Alpha-olefin oligomers and their use as hydraulic fluids and synthetic lubricants (synlubes) are well known. U.S. Pat. No. 2,927,129 reports the oligomerization of C.sub.5-14 .alpha.-olefins using a dialkyl peroxide catalyst to make a synlube. U.S. Pat. No. 3,113,167 describes an .alpha.-olefin oligomer process using a titanium halide and an aluminum compound as the oligomerization catalyst.
The preferred catalysts for making .alpha.-olefin oligomers are Friedel-Crafts catalysts such as boron trifluoride (BF.sub.3) as disclosed in U.S. Pat. No. 3,149,178. Optimum properties are obtained starting with 1-decene although mixtures of .alpha.-olefins have been used, cf. U.S. Pat. No. 3,330,883.
The preferred Friedel-Crafts catalyst is BF.sub.3. Pure BF.sub.3 is not an effective oligomerization catalyst. A small amount of polar compound is necessary as a promoter which forms a complex with the BF.sub.3. U.S. Pat. No. 3,382,291 describes the user of alcohol promoters such as decanol. Other reported promoters are mordenite (hydrogen form), water, phosphoric acid, fatty acids (e.g., valeric acid), ketones, organic esters, ethers, polyhydric alcohols, silica gel and the like.
While the BF.sub.3 based catalysts are preferred, they are expensive and, in many cases, not easily reused and must be disposed of, which in itself presents a significant problem. Thus, it would be beneficial to be able to recover BF.sub.3 values in a manner which would allow for catalyst recycle.
The product stream from the olefin oligomerization is a liquid and contains a mixture of .alpha.-olefin oligomers, dissolved BF.sub.3 and the BF.sub.3 and promoter complex. The dissolved BF.sub.3 is not present in a great amount but can be recovered by any of several different recovery methods. Vogel, et al., U.S. Pat. No. 4,454,366 and U.S. Pat. No. 4,384,162, describe the use of polyvinyl alcohol to remove BF.sub.3 from an oligomerization reaction. Vogel, et al., U.S. Pat. No. 4,433,197, contacts the reaction product with silica to remove the BF.sub.3. Morganson, et al., U.S. Pat. No. 4,429,177, and Madgavkar, et al., U.S. Pat. No. 4,213,001 and U.S. Pat. No. 4,308,414, use silica as an absorbent for BF.sub.3 in an oligomerization process. Madgavkar, et al., U.S. Pat. No. 4,394,296, describe the use of wet silica as a co-catalyst with BF.sub.3 in an oligomer process. The silica can be filtered off and recycled as the catalyst. Madgavkar, et al., U.S. Pat. No. 4,263,467, remove BF.sub.3 by trickling the reaction product over an inert metallic or ceramic bed whereby the BF.sub.3 is said to evaporate and can be recovered. Tycer, et al., U.S. Pat. No. 4,981,578, teaches the recovery of BF.sub.3 by contacting the oligomer product stream with solid or aqueous KF, NaF or NH.sub.4 F. Walker, et al., U.S. Pat. No. 4,956,513, teaches BF.sub.3 recovery by extracting BF.sub.3 from the oligomer reaction product by washing same with water.
From this it can be seen that a great deal of effort has gone into developing a method for removing BF.sub.3 from an olefin oligomerization process in an environmentally safe manner.
The Invention
This invention relates to a process for the recovery of BF.sub.3 from a promoted BF.sub.3 catalyzed .alpha.-olefin oligomerization product stream. The recovered BF.sub.3 can come from that which is dissolved in the liquid product stream and that which is present as a BF.sub.3 /promoter complex. The process features the thermal cracking of at least a portion of the BF.sub.3 /promoter complex to yield promoter and gaseous BF.sub.3. The BF.sub.3 gas is effervescent to the extent that it is not dissolved in the stream. With regard to the dissolved BF.sub.3, be it from thermal cracking or otherwise, its concentration in the stream will be attenuated as the thermal cracking heats the stream to thereby increase the vapor pressure of the dissolved BF.sub.3 so that at least a portion thereof leaves the stream as a gas.
The gaseous BF.sub.3 is recoverable for reuse by quenching the gas with a liquid .alpha.-olefin stream. Preferably, the olefin stream will be the same as that used to feed the oligomerization reaction. The quenching results in a portion of the gaseous BF.sub.3 being dissolved in the stream. The benefits from such a quenching are limited by the solubility of the BF.sub.3 in the olefin stream. It is well recognized that such solubility is rather small. As a result, not much of the gaseous BF.sub.3 is recovered in this manner.
Instead, it is preferred to quench the gaseous BF.sub.3 with a liquid olefin stream which contains a promoter. With this technique, there is obtained a much greater B.sub.3 presence in the stream. The promoter forms a complex with the BF.sub.3, which complex can be present in the olefin stream in an amount which is much greater than that which can obtained by simply dissolving BF.sub.3 in the stream. Thus, much of the gaseous BF.sub.3 can be reused and there can be obtained an .alpha.-olefin stream which contains dissolved BF.sub.3 and the BF.sub.3 /promoter complex and which, as a result, is suitable for feed to an olefin oligomerization reaction.